A 12-month-old child is brought to the immunisation clinic for the 12-month vaccination visit as per the National Immunisation Schedule. The mother reports that the child had severe allergic reaction (urticaria and angioedema) after the first dose of DPT vaccine at 6 weeks. The child is otherwise healthy and has received all other scheduled vaccines without adverse events. Which investigation is most appropriate to determine if the child can safely receive the second dose of DPT or if an alternative vaccine formulation is required?

Explanation

## Investigation of Choice for DPT Hypersensitivity Assessment ### Clinical Context A history of severe allergic reaction (urticaria and angioedema) after DPT vaccine raises concern for true IgE-mediated hypersensitivity. Before proceeding with the next dose, it is essential to identify the specific component causing the reaction and assess whether the child can tolerate the vaccine or requires an alternative formulation. ### Why Intradermal Skin Test? **Key Point:** Intradermal skin testing with DPT vaccine (or individual components: diphtheria toxoid, tetanus toxoid, whole-cell or acellular pertussis) is the gold standard for identifying IgE-mediated hypersensitivity and determining which component(s) are responsible for the reaction. **High-Yield:** - **Mechanism:** Detects IgE antibodies bound to mast cells in the skin - **Sensitivity & Specificity:** High for IgE-mediated reactions (urticaria, angioedema, anaphylaxis) - **Practical advantage:** Can test individual vaccine components (e.g., acellular pertussis vs. whole-cell, specific toxoids) - **Clinical utility:** Guides choice of alternative vaccine (e.g., acellular pertussis if whole-cell is culprit; toxoid-only if pertussis component is responsible) ### Interpretation & Management | Skin Test Result | Interpretation | Management | |---|---|---| | Positive (wheal + flare) | IgE-mediated hypersensitivity to specific component | Avoid that component; use alternative formulation (e.g., DT without P) | | Negative | No IgE-mediated allergy; prior reaction likely non-IgE (e.g., coincidental infection) | Can safely give DPT; monitor closely | **Clinical Pearl:** In India, if the child is allergic to pertussis component, DT (diphtheria-tetanus) vaccine alone can be given to maintain protection against diphtheria and tetanus, as per IAP guidelines. Pertussis vaccination can be deferred or given as acellular formulation if available. ### Why Intradermal Over Serology? - **Serum IgE** is non-specific and often elevated in atopic individuals without clinical relevance - **Skin test** directly detects biologically active, cell-bound IgE and is more clinically predictive - **Serum IgG** (option 3) measures prior immunity, not allergy; it does not guide safety of re-vaccination [cite:IAP Immunisation Schedule 2023; Park 26e Ch 7]